The invention relates to a particle beam system having a mirror corrector. A system of this kind is, for example, disclosed in U.S. Pat. No. 5,319,207. The mirror corrector functions in such systems to correct the geometric and energy-dependent aberrations of the particle-optical components contained in the system.
Mirror correctors include a magnetic beam deflector in addition to an electrostatic mirror. Such a beam deflector functions, to a certain extent, as a switch for the particle beams in order to, on the one hand, deflect the particle beam exiting from a source to the electrostatic mirror and to deflect the particle beam, which is reflected at the electrostatic mirror, to the downstream imaging optics.
So that the beam deflector, being a non-rotational-symmetrical electron-optical component does not itself generate aberrations of the second order, it is known from U.S. Pat. No. 5,319,207 to symmetrically configure corresponding beam deflectors so that the beam deflector has two symmetry planes which are perpendicular to the beam paths of the particle beams and simultaneously lie in the bisecting line of the deflections achieved in the individual regions of the deflector. Within the beam deflector, a course of the fundamental paths of the particle beam symmetrical to the symmetry planes is achieved because of the above symmetry in the configuration of the beam deflector and the simultaneous imaging of the symmetry planes onto each other via a mirror or a combination of a mirror and a field lens. In this way, the aberrations of the second order vanish within the beam deflector. So that this symmetrical course of the beam paths within the deflector is ensured, it is, however, necessary that the electrostatic mirror, on the one hand, is mounted conjugated to the symmetry planes of the deflector and, on the other hand, simultaneously images the symmetry planes onto each other at an imaging scale of 1:1.
If in this deflector of only two quadratic sector magnets an intermediate imaging plane is imaged into the symmetry planes, is then there results a simple and short configuration but combination aberrations result because of the large dispersion in the mirror and these combination aberrations can be corrected only to a limited extent. If, in contrast, the diffraction plane of the objective lens is imaged into the symmetry plane of the deflector, then such combination aberrations do not occur because of a vanishing dispersion in the mirror. However, in this operating mode, a dispersion in the image occurs which must be compensated after a two-time passthrough through the deflector. The large focal length of the deflector then, however, requires a reduction of the beam diameter which is realizable only with very long lengths or at least two-stage objective lens systems and likewise at least two-stage mirror systems.
In U.S. Pat. No. 5,319,207, deflectors are already described which are dispersion-free for a single passthrough of the particle beam. These deflectors, however, include either three different magnetic fields having an additional superposed electrostatic field or magnetic sectors having partially concave outer surfaces. The concave outer surfaces of the magnetic sectors, however, require a corresponding concave formation of the magnetic coils whereby, in turn, also manufacturing problems occur, especially in a series production.